The present invention relates to amorphous perfluorinated or fluorinated polymers substantially unstable ionic end group free, in particular COF, COOH or their corresponding esters, or salts or said end groups being undetectable by the method reported hereunder.
More specifically the present invention relates to amorphous (per)fluorinated polymers containing cyclic perfluorinated structures.
Said polymers are characterized by a high transparency at wave lengths from 150 to 250 nm. Therefore said polymers are usable for achieving protective films in the production of semiconductors by means of microlithographic techniques at 248 nm, 193 nm and 157 nm.
It is known that amorphous fluorinated polymers when used for microlithographic applications must show the lowest possible absorption with respect to the wave lengths of the incident light. In this application fluoropolymers are required having transparency at lower and lower wave lengths, from 248 nm to 193 nm and preferably even to 157 nm, to have smaller and smaller and quicker and quicker chips.
The amorphous fluorinated polymers are characterized by a high transparency in a wide range of wave lengths, however at wave lengths lower than 250 nm the transparency is not high. This is mainly due to the fact that the amorphous polymers, obtained by the known conventional syntheses of the prior art, contain unstable polar ionic end groups, mainly of the COF, COOH type, which absorb at wave lengths lower than 250 nm, reducing the film transparency of the amorphous (per)fluorinated polymer to said wave length ranges.
Various processes to decrease or neutralize the residual amounts of said polar end groups are known in the prior art, however the known methods do not allow to lead to a substantial elimination of ionic end groups, in particular the COF and COOH end groups.
One of the methods used to neutralize the acid end groups in polymers is by fluorination: the fluorinating agent is generally elementary fluorine, but also other fluorinating agents are used.
The polymer can be fluorinated under solid form as described in U.S. Pat. No. 4,743,658, or dissolved in solvents which are stable to fluorination, as described in EP 919,060. Both treatments are carried out at high temperatures, particularly of the order of 200xc2x0 C., with fluorine diluted with inert gas. Or, before fluorination a pre-treatment of the end groups can be carried out with tertiary amines or alcohols to favour the subsequent fluorination reaction. The temperatures are in the range 75xc2x0 C.-200xc2x0 C. and must be lower than the polymer Tg. See patent application WO 89/12,240 and U.S. Pat. No. 4,966,435.
By said methods of the prior art a reduction of the polar end groups is obtained but not their substantially complete elimination. Besides, in some cases during the process other polar end groups such as for example COF are formed. See the comparative Examples.
The residual presence of polar end groups in amorphous (per) fluorinated polymers, as said, worsens their optical properties and compromises the use thereof in the microlithography field, in particular at wave lengths lower than 200 nm, more particularly lower than 180 nm.
The Applicant has unexpectedly and surprisingly found perfluorinated or fluorinated amorphous polymers substantially unstable ionic end group free, in particular COF, COOH, or their esters, or salts, on the basis of the analysis method reported hereunder. An object of the present invention are amorphous perfiuorinated or fluorinated polymers substantially unstable ionic end group free, in particular COF, COOH or their esters or salts, said end groups being undetectable by the method reported below, i.e. each end group being in a total amount lower than 0.05 mmoles/Kg polymer; the determination method of acid end groups being the Fourier transform IR spectroscopy by Nicolet(copyright) Nexus FT-IR equipment (256 scannings, resolution 2 cmxe2x88x921), wherein on a sintered polymer powder pellet having a 5 mm diameter and thickness from 50 to 300 microns (1.75-10.5 mg of polymer) a scanning between 4,000 cmxe2x88x921 and 400 cmxe2x88x921 is initially carried out, the pellet being then kept for 12 hours in an environment saturated with ammonia vapours, and finally recording the IR spectrum under the same conditions of the initial IR spectrum; elaborating the two spectra by subtracting from the signals of the spectrum relating to the untreated sample (initial spectrum) the corresponding ones of the specimen spectrum after exposure to ammonia vapours, obtaining the udifferencen spectrum, which is normalized by the following equation:       “          Difference      ⁢              xe2x80x83            ⁢      spectrum        ”        [          pellet      ⁢              xe2x80x83            ⁢      weight      ⁢              xe2x80x83            ⁢                        (          g          )                /        pellet            ⁢              xe2x80x83            ⁢      area      ⁢              xe2x80x83            ⁢              (                  cm          2                )              ]  
The optical densities related to the end groups which have reacted with the ammonia vapours are determined; said end groups being the COOH and COF end groups, that with ammonia vapours give rise to detectable peaks; the optical densities are converted in mmoles/kg polymer using the extinction coefficients reported in Table 1, page 73 of the paper by M. Pianca et Al. xe2x80x9cEnd groups in fluoropolymersxe2x80x9d, J. Fluorine Chem. 95 (1999), 71-84 (herein incorporated by reference); the found values express the concentrations of the residual polar end groups as mmoles of polar end groups/kg of polymer: in the spectrum of the amorphous (per)fluorinated polymers bands related to COOH groups (3,600-3,500, 1,820-1,770 cmxe2x88x921) and/or COF groups (1,900-1,830 cmxe2x88x921) are not detectable, the method detectability limit being 0.05 mmoles/Kg polymer.
More particularly the present invention relates to amorphous (per) fluorinated polymers containing cyclic perfluorinated structures.
With amorphous polymers according to the present invention, besides the properly said amorphous polymers, also semicrystalline polymers are meant, provided that they are soluble in perfluorinated solvents for at least 1% by weight at temperatures in the range 0xc2x0 C.-100xc2x0 C., preferably 20xc2x0 C.-50xc2x0 C. The completely amorphous polymers show only glass transition temperatures but not melting temperatures; semicrystalline polymers show one glass transition temperature and melting temperatures.
As perfluorinated solvents, perfluoroalkanes, perfluoropolyethers, preferably having boiling point lower than 200xc2x0 C., such as for example Galden(copyright) LS165, tertiary perfluoroamines, etc., can for example be mentioned.
The amorphous polymers according to the present invention contain one or more of the following fluorinated comonomers:
C2-C8 perfluoroolefins, such as tetrafluoroethylene (TFE) hexafluoropropene (HFP);
C2-C8 chloro-fluoroolefins, such as chlorotrifluoroethylene (CTFE);
CF2=CFORf (per)fluoroalkylvinylethers (PAVE), wherein Rf is a C1-C6 (per)fluoroalkyl, for example CF3, C2F5, C3F7;
CF2=CFOX (per)fluoro-oxyalkylvinylethers, wherein X is: a C1-C12 alkyl, or a C1-C12 oxyalkyl, or a C1-C12 (per) fluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl;
fluorosulphonic monomers, preferably selected from the following:
F2C=CFxe2x80x94Oxe2x80x94CF2-CF2xe2x80x94SO2F;
F2C=CFxe2x80x94Oxe2x80x94[CF2xe2x80x94CXFxe2x80x94O]nxe2x80x94CF2xe2x80x94CF2xe2x80x94SO2F wherein X=Cl, F or CF3; n=1-10
F2C=CFxe2x80x94Oxe2x80x94CF2xe2x80x94CF2xe2x80x94CF2xe2x80x94SF;
fluorodioxoles, preferably perfluorodioxoles;
non conjugated dienes of the type:
CF2=CFOCF2CF2CF=CF2,
CFX1=CX2OCX3X4OCX2=CX1F
wherein X1 and X2, equal to or different from each other, are F, Cl or H; X3 and X4, equal to or different from each other, are F or CF3, which during the polymerization cyclopolymerize.
Among fluorodioxoles the ones of formula: 
can be mentioned, wherein Rxe2x80x2F is equal to F, RF or ORF wherein RF is a linear or branched when possible perfluoroalkyl radical having 1-5 carbon atoms; X1 and X2 eual to or different from each other being F or CF3.
Preferably in formula (IA) Rxe2x80x2F=ORF, RF preferably is CF3; X1=X2=F, and the compound is herein indicated as TTD.
When cyclic monomers or monomers which in polymerization generate cyclic structures are present, the amount of said monomers generally ranges from 15 to 100% by moles; preferably from 25% to 100%.
When the cyclic monomer is TTD, the amount % by moles in the polymer ranges from 40 to 95%.
The comonomers which can be copolymerized with cyclic monomers or which cyclize in polymerization are selected from one or more of the following: TFE, chlorotrifluoroethylene (CTFE), hexafluoropropene (HFP), perfluoroalkylvinylethers or perfluorooxyalkylvinylethers as above defined.
The preferred copolymers according to the present invention are the copolymers of TTD with tetrafluoroethylene, the other comonomers when present are generally in amounts ranging from 0% by moles to 20% by moles, preferably lower than 10% by moles.
The TTD dioxoles and the respective homopolymers and copolymers according to the present invention are prepared for example according to U.S. Pat. No. 5,498,682 and U.S. Pat. 5,883,177.
Other amorphous polymers which can be used according to the present invention not containing cyclic structures are based on TFE and HFP, optionally containing perfluorovinylethers as above defined, preferably perfluoromethylvinylether (PMVE), perfluoroethylvinylether (PEVE), perfluoropropylvinylether (PPVE).
The amorphous polymers of the present invention can be prepared according to polymerization methods in emulsion, preferably in microemulsion, in suspension or in bulk according to known methods of the prior art. In particular the amorphous polymers of the present invention can be prepared by copolymerization of monomers in aqueous emulsion, according to known methods of the prior art, in the presence of radical initiators, for example persulphates, perphosphates, alkaline or ammonium perborates or percarbonates, optionally in combination with ferrous, cuprous or silver salts, or of other easily oxidizable metals. In the reaction medium also surfactants of various type are usually present, among which fluorinated surfactants are particularly preferred. The polymerization reaction is generally carried out at temperatures in the range 25xc2x0-150xc2x0 C., under pressure up to 10 MPa. The preparation is preferably carried out in microemulsion of (per)fluoropolyoxyalkylenes, according to U.S. Pat. No. 4,789,717 and U.S. Pat. No. 4,864,006. Optionally in polymerization also well known chain transfer agents of the prior art can be used.
A further object of the present invention is a process for preparing the amorphous (per)fluorinated polymers of the present invention, containing reduced amounts or substantially ionic end group free as above defined, by treatment with elementary fluorine, optionally in admixture with inert gases, in a solvent inert to fluorination, in the presence of ultraviolet radiations having wave length from 200 to 500 nm, operating at temperatures lower than 100xc2x0 C.
The used radiation has a wave length ranging from 200 to 500 nm, emitted for example from a mercury lamp Hanau TQ 150.
The reaction temperature preferably ranges from 0xc2x0 C. to +100xc2x0 C., preferably from +20xc2x0 C. to +50xc2x0 C.
Preferably the polymer concentration in the perfluorinated solvent is in the range 1-11% by weight.
At the end of the fluorination the solvent can be recovered by distillation and suitably reused.
As said, the determination of the acid end groups before and after the fluorination is carried out by IR spectroscopy, performing a scanning between 4,000 cmxe2x88x921 and 400 cmxe2x88x921, on a sintered polymer powder pellet which can have a thickness from 50 to 300 micron. The fluorination process ends when by IR spectroscopy bands relating to COOH groups (3,600-3,500, 1,820-1,770 cmxe2x88x921) and/or COF groups (1,900-1,830 cmxe2x88x921) are no longer detectable. The method detectability limit is 0.05 mmoles/Kg polymer.
As said, the polymers of the invention are characterized by a high transparency in a wide range of wave lengths, in particular from 150 to 250 nm. Therefore said polymers are usable for achieving transparent protective films in the semiconductor production by microlithographic techniques in the above wave length range and in particular at 248 nm, 193 nm and 157 nm.
The protective films are applied by casting, spin coating or other conventional methods.
The following Examples illustrate the invention and do not limit the scope thereof.